The present invention relates to a method and an arrangement comprising an epicyclic gearbox for driving two vehicle axles of a motor vehicle, in which two inner central wheels with different numbers of teeth and engaging with a planet wheel are respectively in permanent drive connection with one vehicle axle and a planet carrier of the planet wheel can be locked solid via a centrifugal brake.
In a known driving arrangement as shown, for example, in ATZ Automobiltechnische Zeitschrift 89 (1987), No. 11, p. 596, the front axle is permanently driven and the epicyclic gearbox with the centrifugal brake is used to switch the rear axle steplessly on and off exclusively as a function of the slip when slip appears.
It is necessary to provide safety with respect to the bursting of the epicyclic gearbox if the associated control electronics should fail and, consequently, high rotational speeds in the epicyclic gearbox are the result of slip occurring. An object on which the present invention is, therefore, based is to be able both to reduce the torque of a friction clutch effectively located between the two vehicle axles by way of the epicyclic gearbox and also to adjust it steplessly as a function of the slip of one vehicle axle.
Such friction clutches can be used for switching on one vehicle axle in the case of an all-wheel drive system in which the other vehicle axle is permanently driven. The referenced friction clutches are also employed for the stepless locking of an epicyclic distributor gearbox effectively located between the two vehicle axles.
The foregoing object has been achieved in an advantageous manner in accordance with the present invention in that the planet carrier is operatively connected to one of the inner central wheels by a friction clutch whose torque is controllable as a function of slip occurring at one wheel of a vehicle axle and, with an automatic lock preventer (ABS) present, a response rotational speed of the centrifugal brake is sufficiently high to avoid suppression of slip threshold values of the lock preventer (ABS).
In an embodiment according to the present invention, the planet carrier connected to the centrifugal brake is driven more rapidly on the appearance of differential rotational speed between the inner central wheels so that the clutch torque of the friction clutch is reduced. The planet carrier does not, however, enter a critical rotational speed range in which there is a danger of the epicyclic gearbox bursting in the event that the functions of the control electronics and the electromagnetic clutch setting elements do not fail, i.e. provided the rotational speed of the planet wheel carrier is limited by the friction clutch.
If, unfortunately, slip should occur on one vehicle axle after the failure of the control electronics or of the clutch setting element, the planet carrier accelerated to high rotational speeds is retarded by the centrifugal brake so that the danger of bursting is avoided.
Another advantageous aspect of the drive arrangement according to the present invention is that it has use in an all-wheel drive system with a vehicle axle which can be switched on. In such a drive arrangement, the use of an input bell housing for connecting the input shaft both with the friction clutch and with a coaxial output shaft has been found to be advantageous.
Yet another advantageous aspect of the drive arrangement according to the present invention is its use in an all-wheel drive system in which both vehicle axles are permanently driven, and the two central wheels and the input shaft driven by an engine are connected to one another by an epicylic distributor gearbox.
In one embodiment according to the invention, light-weight construction and rapid response behavior are provided to a particular degree by the use of a plate clutch and/or an electromagnetic setting element for the control of the clutch.
The use of the arrangement according to the present invention in a motor vehicle equipped with an automatic lock preventer is made possible in an advantageous manner by providing that the friction clutch is disengaged when the brakes are actuated.
An operationally reliable friction lock of a distributor gearbox or the switching on of a vehicle axle with small clutch forces is achieved in the arrangement according to the present invention because the necessary clutch torques are reduced by the epicyclic gearbox.
In the case of spin of only one vehicle axis due to drive torques which cannot be reduced or the locking of only one vehicle axis due to braking torques which cannot be reduced, high differential rotational speeds arise which could lead to the destruction of the epicyclic gearbox and clutch and which also represent a danger for the occupants of the vehicle.
In normal operation, unallowable differential rotational speeds are prevented because the drive torques and braking torques of the engine are distributed in the correct ratio to the two vehicle axles by actuation of the clutch and an anti-lock braking system limits the brake slip of each vehicle axle to an allowable maximum amount by reducing the brake pressure.
In the arrangement according to the present invention, the clutch is not constructed as a pure centrifugal clutch. This has been recognized to be disadvantageous because the threshold value for the drive slip of the engine is lower than that for the brake slip. If such a clutch were to engage in the case of a full braking operation at the same low slip value as is necessary in the drive case, the front axle, which has to run in a higher slip range for the purpose of good deceleration, would also retard the rear axle via such a clutch. This, however, reduces the important lateral guidance of the rear axle and impairs the function of the anti-lock braking system.
An essential advantage of the arrangement according to the present invention consists in the fact that the clutch is configured as an electromagnetically actuated plate clutch so that switching on can be controlled in accordance with the particular requirements and so that this clutch, together with the epicyclic gearbox, can be protected from bursting by a centrifugal brake in the event of failure of the electronics. The threshold value for the centrifugal brake is placed sufficiently high to permit full anti-lock braking without any influence thereon.
The centrifugal brake can be configured in such that there is a longitudinally slotted drum which is fastened to the planet carrier and whose straps can spring out radially under centrifugal force, come into contact with the casing and thereby retard the planet carrier via frictional connection.
An essential advantage of the invention is that anti-lock braking operations are included in the working range of the arrangement unlike known driving arrangement which does not permit anti-lock braking because in it a high differential rotational speed occurs between the front axle and the rear axle. Thus, the front axle can run in a high slip range for the purpose of good retardation values; the rear axle, in contrast, runs in a low slip range for the purpose of good lateral guidance. The centrifugal brake of the known arrangement always limits the differential rotational speed to the desired drive slip which, however, is much smaller than the brake slip necessary for anti-lock braking.
In the arrangement according to the invention, the drive working range and the brake working range are distinguished from each other. The friction clutch permits correspondingly different differential rotational speeds. The centrifugal brake of the arrangement according to the invention only represents a device to protect the epicyclic gearbox from bursting should an electronic failure occur (an anti-lock braking failure and an emergency braking action in which the front axle locks but the rear axle does not would be conceivable).
In the arrangement according to the invention, the response rotational speed of the centrifugal brake is sufficiently high for the slip threshold values of the lock preventer so as not to be suppressed.
The determination of the slip threshold value in an anti-lock braking system (ABS) as an embodiment of a lock preventer is known per se as shown in Vogel-Fachbuch Technik Kraftfahrzeugwesen, Volume Fahrwerktechnik: Fahrverhalten; Vogel Buchverlag Wurzburg, 1st edition 1987, p. 94, FIG. 3.36.